Conventionally, transfer molding or injection molding has been used for molding a resin. These techniques use a mold for molding a resin. The mold is provided with a resin flow channel and a cavity. A fluid resin is injected into the cavity through the resin flow channel. When the fluid resin in the cavity is cured, a cured resin is formed. As a result, a molded body having the cured resin is completed.
A thermosetting resin is used as the fluid resin described above, and tool steel is used as a material for the mold. In this case, an adhesion property between the cured resin and the surface of the mold (mold surface) should be reduced for easy removal of the molded body. In other words, releasability between the cured resin and the mold surface should be improved.
As a surface reforming material facilitating release of a mold from a cured resin, an organic material such as polytetrafluoroethylene or silicone rubber having a good non-wetting (dewetting) characteristic, that is, a low wetting characteristic, with respect to a fluid resin is considered to be promising. A method of molding a resin using these organic materials is disclosed in Japanese Patent Laying-Open No. 07-329099 (on page 3 to page 4). In this method, firstly, the organic material described above is sprayed or applied onto a mold surface. Thereafter, the organic material is dried, and thus coating of the organic material on the mold surface is completed.
Further, there may be a case where a chip-type electronic component such as an LSI chip mounted on a lead frame, a printed board, or the like (hereinafter will be referred to as a “chip”) is sealed with a resin. In this case, a thermosetting resin containing a ceramic filler, for example, an epoxy resin is used as a fluid resin. Since the filler wears the mold surface, a method of forming on the mold surface an inorganic material with high hardness having wear resistance is employed. In the method, the mold surface is coated with an inorganic material with high hardness excellent in wear resistance such as Cr, TiC, or CrN, by means of plating, PVD (Physical Vapor Deposition), CVD (Chemical Vapor Deposition), or the like.
Also, Japanese Patent Laying-Open No. 2004-25677 proposes a method of emitting a gas component contained in a fluid resin out of a mold for molding a resin made of a porous material having a three-dimensional communicating hole, through the communicating hole (on page 5 to page 6, and in FIGS. 1 and 2).
However, the conventional technique described in the foregoing Japanese Patent Laying-Open No. 2004-25677 (on page 5 to page 6, and in FIGS. 1 and 2) has problems as described below.
Firstly, when the mold is made of a conventional material, a cured resin is likely to stick to a mold surface. For this reason, the mold surface should be cleaned periodically to allow the cured resin to be always easily removed from the mold. Therefore, frequent maintenance is required.
Secondly, numerous eject mechanisms are required to remove a molded body from the mold. Thus, the mold becomes larger and has a complicated structure.
Thirdly, when the mold surface is coated with an organic material such as polytetrafluoroethylene or silicone rubber, these organic materials are likely to be worn by a filler contained in the fluid resin. Therefore, it is difficult to use these organic materials singly as a material for reforming the mold surface.
Fourthly, when the mold surface is coated with an inorganic material with high hardness excellent in wear resistance such as Cr, TiC, or CrN, releasability between the cured resin and the mold surface is insufficient because these inorganic materials do not have a sufficient non-wetting characteristic with respect to a fluid resin. Further, when the mold for molding a resin is made of a porous material, the range of selection for a material excellent in releasability is narrow.
Furthermore, there is a problem resulting from the fact that a method of evaluating releasability, in other words, a method of evaluating an adhesion property of a mold surface with respect to a cured resin has not been established. To explain this problem, two processes assumed as processes through which a cured resin sticks to a mold surface will now be described.
A first process is a process in which a silane coupling agent contained in a fluid resin made of a thermosetting resin plays a role in the sticking of a cured resin to a mold surface. The silane coupling agent is added to the thermosetting resin. The silane coupling agent contains an alkoxy group (alkoxyl group). The alkoxy group reacts with a hydroxyl group existing in the surface of a filler. Thereby, an alcohol is produced, and the alkoxy group is chemically absorbed into the surface of the filler. Therefore, an absorption layer is formed in the surface of the filler. As a result, a wetting characteristic between the filler and the fluid resin is enhanced due to the absorption layer.
The chemical absorption described above occurs not only in the surface of the filler but also in the surface of an oxide layer of the mold surface. In this case, firstly, moisture in the air is absorbed into the surface of the oxide layer existing in the mold surface, and thus a hydroxyl group is formed in the surface of the oxide layer. Next, the hydroxyl group reacts with an alkoxy group, and thus an absorption layer is formed in the surface of the oxide layer. As a result, a wetting characteristic between the mold surface and the fluid resin is enhanced. Next, the thermosetting resin constituting the fluid resin is heated to form a cured resin. In this occasion, the sticking strength between the mold surface and the cured resin is increased due to the enhanced wetting characteristic between the mold surface and the fluid resin.
A second process is a process in which an amine-based curing agent contained in a fluid resin made of a thermosetting resin plays a role in the sticking of a cured resin to a mold surface. In this process, the amine-based curing agent reacts with a hydroxyl group in the surface of an oxide layer to form an amine complex. Thereafter, curing of the thermosetting resin on the surface of the oxide progresses due to the presence of the amine complex. As a result, the oxide layer and the thermosetting resin firmly stick together via the amine complex.
In the two processes described above, a change occurs in releasability between the cured resin and the mold surface, more generally speaking, an adhesion property between the cured resin and the surface of a member. As a physical factor of the change in the adhesion property, there is known a possibility that surface roughness is involved in the change. Further, as a chemical factor of the above change, there is known a possibility that hydrogen bond is involved in the change. However, a method of strictly evaluating an adhesion property between a cured resin and the surface of a member has not been established yet. Therefore, in a conventional method of evaluating an adhesion property, samples of a plurality of oxides should be prepared and tested individually, and thus there is a problem that the method requires much time and effort.
Patent Document 1: Japanese Patent Laying-Open No. 07-329099 (page 3 to page 4)
Patent Document 2: Japanese Patent Laying-Open No. 2004-25677 (page 5 to page 6, FIGS. 1 and 2)